A fiber reinforced plastic comprising reinforcing fibers and a matrix resin generally have a high specific strength, high specific modulus and good mechanical characteristics, as well as good functional characteristics such as high weather resistance and chemical resistance. Therefore increasing attention has been focused on them in various industrial fields and demands for them are mounting in recent years.
As a molding method for a fiber reinforced plastic having good functional characteristics, there is an autoclave molding method in which prepregs, i.e., semicured basic elements (prepreg sheets) comprising continuous reinforcing fibers impregnated with a matrix resin, are laminated, followed by heating and pressing in a high-temperature, high-pressure vessel for curing of the matrix resin to provide a molded fiber reinforced plastic. The autoclave molding method is used commonly.
As another molding method for a fiber reinforced plastic, there is an RTM (resin transfer molding) method in which, to improve the production efficiency, a basic element (preform) comprising continuous reinforcing fibers and having a form shaped to a form of a member being produced is impregnated with a matrix resin and then cured.
Fiber reinforced plastics produced by these molding methods have good mechanical properties because the reinforcing fibers are in a continuous form. In addition, because the continuous fibers are arranged regularly, the fiber-reinforced plastics can be designed to have required mechanical properties by laminating base materials in proper arrangements. Moreover, the resulting fiber reinforced plastics have small variation in mechanical properties. On the other hand, the use of continuous fibers makes it difficult to produce a product in a complicated shape such as three-dimensional ones, and therefore, these molding methods have been used almost exclusively for producing products in a nearly planar shape.
A molding method called an SMC (sheet molding compound) molding method is useful to produce moldings in a three-dimensional or other complicated shape. In the SMC molding method, chopped strands of reinforcing fibers, commonly having a length of about 25 mm, are impregnated with a thermosetting matrix resin to provide SMC sheets in a semicured state, which will be molded by heating and pressing in a heating-pressing machine. In many cases, SMC sheets are cut before pressing to a size smaller than a form of a molding and put on a mold, and the cut SMC sheets having the small sizes are extended (i.e., made to flow) in the molding method. Thus, the flow of the matrix resin and many cut reinforcing fibers serves to achieve the ability of flexible shaping into a three-dimensional or other shape.
However, the SMC molding method inevitably gives rise to irregular distribution and irregular orientation of chopped strands during the SMC sheet production step, leading to moldings having poor mechanical properties or a large variation in them. Furthermore, such irregular distribution and irregular orientation of chopped strands tends to result in warp, surface sinks, etc., in thin moldings in particular. Such moldings will not serve effectively for some structural uses.
To eliminate such defects in the aforementioned materials, JP 63-247012 A and JP 09-254227 A have disclosed prepreg base materials in a form of a prepreg sheet comprising continuous fibers and a thermoplastic resin in which incisions are made to sever the continuous fibers to allow the fibers to flow and to provide molding having a decreased variation in mechanical properties.
However, although moldings produced from the prepreg base materials having the incisions have largely improved mechanical characteristics with a decreased variation as compared with those produced by the SMC molding method, they cannot be said to have a sufficient strength to serve as structural members. As compared with prepreg base materials comprising continuous fibers, the incisions contained in the prepreg base materials act as defects where stress concentrates to cause destruction in the resulting moldings, causing, in particular, problems such as a decrease in tensile strength and tension fatigue strength of the moldings.
It could therefore be helpful to provide a prepreg base material that comprises reinforcing fibers maintaining a high flowability during a molding process and has a flexible shaping ability to form a molding in a complicated shape. It could also be helpful to provide a laminated base material comprising reinforcing fiber sheets at least some of which are the prepreg base material and a fiber-reinforced plastic that are produced by molding a laminated base material. It could still further be helpful to provide a process to produce the prepreg base material and a process to produce a fiber reinforced plastic from the prepreg base material.